The central goal of this investigation is to examine the relationship between the display of specific cell surface proteins and cellular senescence (aging). Age-specific membrane proteins will be characterized and isolated. The isolated species will be used as antigens to produce specific antisera. The antisera will then provide a means to identify the in vitro age of other cells (via indirect immunofluorescence) and to construct a model that correlates in vitro age with specific surface markers. In addition, antiserum that is specific for the surface antigens of early passage, late passage, or post-replicative cells will be used to select (via complement mediated cytotoxicity) for subpopulations of cells that are homogeneous with regard to age. It is proposed that the development of such a selection technique will become a useful tool with which to examine the regulation of cell proliferation.